The present invention relates to a servo circuit suitable for use in an optical disk player or the like.
FIG. 2 indicates a servo byte pattern of a sample servo employed in an optical disk player. Each sector of the optical disk is made up of 43 servo blocks, in which one servo block is composed of two servo bytes followed by 16 data bytes. Each servo byte is formed by two wobbled pits and one clock pit. The two wobbled pits are disposed to the right and left with respect to the track center. Thus, when the pickup (data detecting light spot) traces the track center, the amount of light reflected from the right and left wobbled pits decreases equally. Displacement of the tracing position in the rightward or leftward direction changes the relative amounts of light reflected from the two wobbled pits corresponding to the direction of displacement and the distance of displacement. A tracking error signal can be produced from the difference between the amounts of light reflected from the pairs of wobbled pits, as indicated by the level difference between the RF signal in the two regions, and is employed in the following data byte region.
The arrangement of the two wobbled pits is varied so that a longer interval and a shorter interval between the two wobbled pits are alternately changed at every 16 tracks. Thus, the detection of the change in the interval between the pairs of wobbled pits makes it possible to accurately count the number of tracks (16-track counting), even during high speed search operations.
The distance D between the rearward wobbled pit and the clock pit is set to a specific length which does not appear in the data bytes. Therefore, this distance D can be detected and used as a synchronizing signal, and various timing signals produced on the basis of the detected synchronizing signal. Clock pulses are produced corresponding to the detection signals of clock pits. A mirror-surface portion of the distance D is a focusing area in which a focus error signal is detected and employed during the following data byte region.
In an optical disk player for such an optical disk, a tracking servo loop switch is rendered open when the level of the tracking error signal exceeds a predetermined reference level in order to protect the tracking actuator. Alternatively, a limiter switch may be rendered ON to limit the level of the tracking error signal so that no error signal above a predetermined level is outputted.
FIG. 1 is a circuit diagram showing a prior art level detector which operates as a protection limiter switch. As shown in the figure, an error signal is applied to the actuator (not shown) and to a window comparator 1. The window comparator 1 compares the error signal with an upper limit reference level V.sub.1 and a lower limit reference level V.sub.2, and outputs an alarm signal when the error signal is above the reference level V.sub.1 or below the reference level V.sub.2. In response to the alarm signal, a protection switch is actuated.
However, since the reference levels V.sub.1 and V.sub.2 are set to large values, and hence the protection operation is not frequently effected, the actuator is driven by a large level error signal. Therefore, a quick detection of the error signal cannot be accomplished. Moreover, it takes considerable time to return to a normal condition. Consequently, an effective protection operation cannot be carried out with the prior art device.
Further, when a tracking error signal is produced upon sampling a level difference of the RF signal corresponding to the two wobbled pits, the sampled value is abnormal if there are defects in the wobbled pits or if there is dust in the vicinity of the wobbled pits. Therefore, in the prior art device, the sampled value is compared with a reference value. When the sampled value is above the reference value, the previously sampled value is held as it is. With such a prior art device, the reference value must be set to a relatively large value in order to enable tracking or a largely eccentric disk. Setting the reference value to a large value adversely results in the sampling of abnormal values so that the tracking servo cannot be achieved in defective servo byte regions. On the other hand, if the reference value is set to a small value, the track of an eccentric disk cannot be traced accurately.